1. Technical Field of the Invention
The present invention relates generally to hybrid vehicles and motor generators. More particularly, the invention relates to a control apparatus for a motor generator of a hybrid vehicle, which can economically shift operation of the motor generator between motor and generator modes, thereby minimizing running cost of the hybrid vehicle.
2. Description of the Related Art
In recent years, hybrid vehicles have been developed for the purpose of improving fuel economy.
Hybrid vehicles generally include an internal combustion engine and an electric system that includes, at least, a motor generator and an electric energy storage device (e.g., a battery).
The motor generator is mechanically connected to the drive train of the vehicle and electrically connected to the electric storage device, so that it can selectively operate in either motor or generator mode.
Specifically, in the motor mode, the motor generator receives electric energy from the electric energy storage device to generate torque, thereby assisting the engine in driving the drive train or driving the drive train by itself; in the generator mode, the motor generator receives torque from the drive train to generate electric energy, thereby recharging the electric energy storage device or powering other electric loads.
In order to minimize running cost (e.g., fuel consumption) of the vehicle, it is required to economically control the motor generator, more specifically, to economically shift operation of the motor generator between the motor and generator modes.
Japanese Patent No. 3537810 (to be referred to as patent document 1 hereinafter) discloses a control method for a hybrid vehicle, according to which an optimal running mode of the vehicle is selected from an engine-driven mode, a motor generator-driven mode, and an engine-and-motor generator-driven mode based on the fuel consumption of the vehicle in each of those modes.
However, the above patent document 1 does not address the issue of how to economically shift operation of the motor generator between the motor and generator modes, though the issue is critical to the minimization of running cost of the vehicle.
Moreover, in the above patent document 1, the equivalent fuel consumption of the motor generator in the engine-and-motor generator-driven mode is determined on the assumption that the motor generator generates electric energy only when the engine runs at maximum efficiency and generates torque only with the electric energy generated thereby. In other words, the availability of electric energy generated by low-cost power generations, such as regenerative braking and cogeneration, is not considered in the determination of equivalent fuel consumption of the motor generator. Accordingly, it is impossible to accurately determine the equivalent fuel consumption of the motor generator. Consequently, chances of the motor generator working as a motor are reduced, thus making it difficult to minimize the running cost of the vehicle.
Japanese Patent No. 3662904 (to be referred to as patent document 2 hereinafter) discloses a drive control system for a hybrid vehicle.
The drive control system is configured to determine the ratio of driving force between the engine and the motor generator based on a comparison between a first and a second parameter. The first parameter is defined as the ratio of a decrease in fuel consumption of the engine to an amount of electric energy consumed by the motor generator; the second parameter is defined as the ratio of an amount of electric energy charged into the electric energy storage device (i.e., a battery in this case) to an increase in fuel consumption of the engine.
However, in the above patent document 2, the first and second parameters are defined in different units, more specifically, in units which are in inverse relation to each other. Therefore, it may be difficult to accurately determine the ratio of driving force between the engine and the motor generator based on the comparison between the first and second parameters.